The present invention relates to a carbon dioxide recovery system, and more particularly, relates to a carbon dioxide recovery system capable of sufficiently reducing the oxygen concentration in carbon dioxide gas recovered from combustion exhaust gas.
To prevent carbon dioxide, which has been named as a global-warming gas, from being released into the atmosphere, in thermal power plants and the like in which fossil fuels are used, there has been carried out a method in which exhaust gas generated by the combustion of the fossil fuel is brought into contact with an absorbing solution to remove carbon dioxide from the combustion exhaust gas, the absorbing solution having absorbed carbon dioxide is heated to release carbon dioxide from the absorbing solution, and the absorbing solution is regenerated. Also, it has been proposed that the recovered carbon dioxide gas be used as a raw material in the chemical industry, or be used as a gas for pressure in an oil field for the enhanced oil recovery (EOR) process.
Unfortunately, since in the combustion exhaust gas from which carbon dioxide (CO2) is recovered, oxygen (O2), which is a combustion residue, always exists in the carbon dioxide gas recovered as described above, and the oxygen usually exists in a concentration of 40 to 200 ppm in the carbon dioxide. This oxygen dissolves in the absorbing solution, or exists as bubbles. In the case which this carbon dioxide gas is used as a raw material in the chemical industry, there arises a problem in that this oxygen poisons the catalyst. Also, in the case in which this carbon dioxide gas is used as an oil field filling gas, there arise a problem in that the carbon dioxide gas causes oxidation or corrosion of equipment, and a problem in that the carbon dioxide gas reacts with hydrogen sulfide in the oil field to precipitate sulfur (solid), among other problems. To prevent these problems from arising, it has been demanded to reduce the oxygen concentration in the carbon dioxide gas to be recovered.
Japanese Patent No. 3663117 describes that by dropping the absorbing solution having absorbed carbon dioxide (hereinafter, referred to as a “rich absorbing solution”) into a vessel filled with a filler, the oxygen in the rich absorbing solution can be removed. Also, Japanese Patent No. 3754275 describes an improved method in which when the rich absorbing solution is dropped into the vessel filled with a filler, a stripping gas is caused to flow.
U.S. Pat. No. 7,829,049 describes that if the rich absorbing solution having absorbed carbon dioxide is heated to 70 to 140° C. and is flushed, nitrogen oxide in the rich absorbing solution is removed, and the nitrogen oxide in the recovered carbon dioxide gas can be reduced to almost zero. It has also been proposed that by this processing, the oxygen in the carbon dioxide gas can also be removed together with nitrogen oxide.